Magnetic resonance force microscopy (MRFM) is a variant of scanning probe microscopy that combines the sensitivity and spatial resolution of atomic force microscopy with the three-dimensional and nondestructive imaging capabilities of magnetic resonance imaging. Sample imaging entails mechanically detecting the interaction between sample spins as they undergo magnetic resonance with a nearby gradient magnet. Magnetic resonance phenomena can be detected and imaged by mounting either the sample or the gradient source on a mechanical oscillator and sensing changes in oscillator motion that are induced by magnetic resonance.
Sidles first conceived of the theory of magnetic resonance force microscopy, with potential application as a molecular imaging technology, in the early 1990's. The theory and potential applications of MRFM imaging are described in several early articles (for example, Sidles et al., "The theory of oscillator-coupled magnetic resonance with potential applications to molecular imaging" Review of Scientific Instruments, vol. 63, pp. 3881-3899 (August 1992)). A review article broadly describing magnetic resonance force microscopy was published after further theoretical development and experimental demonstration of the concept of MRFM imaging (Sidles et al., "Magnetic resonance force microscopy" Reviews of Modem Physics, vol. 67, pp. 249-265 (January, 1995)).
Three US patents have also been issued in the field. In U.S. Pat. No. 5,166,615, Sidles discloses a method for mechanically detecting magnetic resonance. A modulation technique patented by Rugar et al. in U.S. Pat. No. 5,266,896 provides a means for creating signals that are easier to detect. In U.S. Pat. No. 5,619,139, Holezer et al. describes an MRFM device in which a spin at the tip of the gradient magnet undergoes magnetic resonance while in atomic interaction with sample spins, thereby scanning sample surfaces.
In force detected magnetic resonance, the basis for magnetic resonance force microscopy, there is a magnetic force between the sample material and the magnetic tip. If either the sample material or the magnetic tip are affixed to a mechanical oscillator, magnetic resonance in the sample can cause a fluctuating force between the sample and the tip that produces detectable changes in oscillator motion. The oscillator is typically a cantilever supported at the base.
In MRFM devices, the inhomogeneous magnetic field B produced by the magnetic tip serves several purposes. The gradient of this field helps to produce the signal force F(t) ##EQU1##
where M and v are the magnetization and volume of the sample material. In addition, the magnetic field produced by the magnetic tip alters the magnetic polarization field of the sample material. The inhomogeneity in polarization field enables the generation of magnetic resonance in a portion of the sample, providing the spatial resolution necessary to image sample features.
More lengthy descriptions of mechanically detected magnetic resonance and magnetic resonance force microscopy are contained in the references previously cited, notably the patents, the Sidles Review of Scientific Instruments article, and the Sidles Reviews of Modern Physics article.
The use of oscillators with a low spring constant (soft oscillators) and high resonant quality (low intrinsic damping) is preferable for MRFM experiments because these oscillators offer force sensitivity advantages. However, oscillators with a low spring constant and low intrinsic damping have disadvantages as well. Some of the problems encountered with the use of soft oscillators are:
(a) The inherent thermal energy in the oscillators causes vibrations that are often the dominant source of experimental noise. The noise resulting from thermal energy hinders the creation of the MRFM signals and decreases the image resolution of the device. PA1 (b) The MRFM signals are difficult to detect because of the dynamical properties associated with a high resonant quality including long damping time and narrow response bandwidth. PA1 (c) Sample scanning and modulation of the sample polarization field are ineffective with oscillators with low spring constant because the distance between the source of field gradient and the sample of interest cannot be effectively varied. PA1 (d) The oscillator can contact and stick to the sample or other device components because of the motion of soft oscillators. PA1 (e) MRFM instrument subsystems cannot be effectively calibrated when soft oscillators are used.
Previous researchers have attempted to actuate soft cantilevers using a base displacement method. This method is not very effective because the low intrinsic spring constant of the cantilever decouples the motion of the tip from the motion of the base and therefore, all the linear displacement modes of the oscillator or the torsional modes of the oscillator cannot be effectively actuated and controlled. In addition, actuation by base displacement cannot provide a means for collocated feedback control of the oscillator, a technique involving the application of control effort to the oscillator near the location where motion is measured. Collocated feedback control offers dynamic advantages that cannot be exploited by base actuation.
Also, previous methods of detecting magnetic resonance use an analog implementation of feedback controller dynamics. Such analog controllers are difficult to configure and adjust. Furthermore, these analog implementations do not adapt to changing feedback requirements.
Numerous problems in MRFM have remained unsolved, including spurious oscillator vibrations that diminish the spatial resolution of the device, unstable signal detection due to oscillator dynamical properties, contacting and sticking of the oscillator to the sample material or other device components ("snap-in"), and the lack of a method for calibrating instrument subsystems.